The prior art recognizes that improved performance of tools, such as drills, can be obtained by application of ceramic coatings (oxides, carbides, nitrides), polycrystalline diamond coatings, and diamond-like carbon coatings onto the cutting surface of the tools. While polycrystalline diamond coatings deposited by chemical vapor deposition (CVD) exhibit extreme hardness (80-100 Giga Pascals (GPa)), they must be deposited at high substrate temperatures (&gt;700.degree. C.), and they exhibit high surface roughness and often poor adhesion to the substrate. Amorphous diamond-like carbon (DLC) coatings are less hard (10-50 GPa), but can be applied at low substrate temperatures (&lt;400.degree. C.). In addition, DLC coatings are smooth, have a low coefficient of friction, and can exhibit high adhesion strength to the substrate. Therefore, DLC coatings have advantages over CVD diamond coatings for many tool applications.
The following references are indicative of the prior art in chemically vapor deposited (CVD) polycrystalline diamond coatings on drills. In U.S. Pat. No. 5,009,705A, Yoshimura et al. disclose a microdrill bit with a CVD polycrystalline diamond coating. In U.S. Pat. Nos. 5,022,801 and 5,096,736, Anthony et al. discuss high temperature CVD diamond coatings on slotted twist drills. In U.S. Pat. No. 5,256,206, Anthony et al. describe a high temperature CVD reactor suitable for coating drills. In JP 01257196 A2, Ito et al. disclose a method for uniform coating of a drill with CVD diamond by precession motion of the drill during deposition. In EP-470447A1, Anthony et al. disclose a heated tubular reactor for CVD diamond deposition on drill bits and similar tools and in EP-528592A1, Iacovangelo describes a masking technique to produce selected area deposition of CVD diamond onto a twist drill.
The following references are indicative of the prior arc of DLC coatings on drills. In JP0248106, Katsumata discloses a DLC coating on drills. In DD-215922A1, Bollinger et al. describe a method for uniform coating deposition on a twist drill. In this method, an independent direct current (DC) electrical field is used near the substrate, optionally modulated with an alternating current (AC) electrical field to direct the coating ion flow from the source to the cathode substrate. In DD-215923A1, Bollinger et al. describe an apparatus for ion coating a spiral drill containing a positively charged electrode shape, preferably a wire, to direct the ion flux to the drill surface, using a positive shielding electrode between the substrate holder and the electrode shape surrounding the drill, e.g. a helical-shaped anode surrounding the drill. In GB 2122224A1, Goode et al. disclose an ion beam method for applying a hard carbon coating onto tungsten carbide drills. Finally in, JP 611464112A, Tobioka et al. discuss deposition of a sputtered Ti adhesion layer, followed by a plasma-deposited carbon coating on tungsten carbide drills.
The prior art methods have not been able to achieve high quality DLC coatings on drills, especially not on printed circuit board drills, while simultaneously meeting the requirements of a robust, high throughput production process.